This invention relates to vehicles equipped with hydraulic systems for propel and hydraulic release braking for the rapid stopping of the same. More particularly, the present invention concerns a hydraulic circuit for simultaneously destroking a hydrostatic pump and releasing a hydraulic brake.
Hydrostatic transmissions, consisting of a hydraulic pump supplying fluid pressure to a hydraulic motor, are known to be useful for propulsion in a variety of vehicles. However, the stopping of such vehicles has proven a persistent problem. Under some conditions simply destroking the pump to a neutral or zero displacement setting, ceasing flow to the hydraulic motor, is sufficient to halt the vehicle. Unfortunately, when the vehicle has developed significant momentum before being commanded to stop, it may coast for a considerable distance. Similarly, a heavy vehicle descending a steep grade may even continue to accelerate when the pump is destroked. Because the wheels of the vehicle are actually driving the hydraulic motor shaft in this situation, the pump and motor roles are reversed. The pump, now acting as a motor, has been commanded to a neutral or zero displacement setting. At low swashplate angles very high speeds may be generated, because speed is a function of flow divided by displacement. Such high speed conditions may cause internal damage to the hydrostatic transmission (HST) or the engine of the vehicle. Worse yet, operators may find themselves riding a runaway piece of equipment.
To address the coasting and overspeed problems, it is known in the art to equip vehicles with hydraulic service brakes. Service brakes supplement the normal dynamic braking of the HST and insure that the vehicle can be more rapidly stopped in an emergency. One type of conventional brake is spring-activated and hydraulically-released. Hydraulic pressure from the HST or another source normally counteracts the spring force to prevent the brake from being applied. When the hydraulic pressure is drained from the brake release line, the spring is released to apply the brake. Under proper conditions, simultaneously destroking the pump and draining the brake release line is effective to rapidly stop the vehicle.